The present invention pertains to 6-hydroxy-3,4-dihydroquinolinone (“6-HQ”) of formula (I) a known compound that is difficult to prepare on a large scale because of the sluggishness of the reaction by which it is prepared using conveniently accessible starting materials and because of the need to maintain a high reaction temperature throughout the reactor. 6-HQ has commercial importance as a key intermediate in the preparation of cilostazol.
Cilostazol (6-[4-(1-cyclohexyl-1H-tetrazol-5-yl)butoxy]-3,4-dihydro-2(1H)-quinolinone) is used to treat symptoms of intermittent claudication in patients suffering from symptoms of the disease, which include pain and cramping while walking due to reduced blood flow to the legs. Cilostazol has the chemical structure of formula (II). 
Cilostazol is described in U.S. Pat. No. 4,277,479, which teaches that it can be prepared by alkylating the phenol group of 6-HQ with a 1-cyclohexyl-5-(4-halobutyl)-tetrazole of formula (III). 
The first preparation of 6-HQ to appear in the U.S. patent literature is Example 11D of U.S. Pat. No. 3,819,637 (the '637 patent). In Example 11D, 6-HQ is prepared by cyclization of (p-methoxyphenyl)-3-chloropropionamide (“MCPA”). A blend of MCPA and AlCl3 was heated with rapid stirring to produce a melt and then further heated to 150° C. and held at that temperature for half an hour. The melt was then poured into a slurry of cracked ice and hydrochloric acid to decompose the aluminum salts. 6-HQ was collected by filtration, washed with water and recrystallized from methanol. When these reaction conditions are scaled up, the high viscosity of the reaction medium causes temperature control problems. Cool regions form within the reactor and the 6-HQ and MCPA in those regions solidifies. Solidification hinders effective mixing of the reagents. Areas where high concentrations of AlCl3 are caused by inadequate mixing can become “hot spots” where thermal decomposition of the reactant and product occur. When we repeated the '637 process using 3 equivalents of AlCl3, the reaction did not go to completion and we obtained the intermediate product N-(4-hydroxyphenyl)-3-chloropropionamide (“HPCA”) in 28% yield. Though most of the HPCA could be removed by recrystallization from methanol as described in the '637 patent, 6-HQ could not be obtained substantially free of contamination with HPCA. Reducing the amount of AlCl3 used in the reaction reduced the amount of HPCA but also reduced the overall yield of 6-HQ. The reaction time also increased; nevertheless, the rapidity of the melt process with either 2 or 3 equivalents of the catalyst is one of this method's merits.
The '637 process is a Friedel-Crafts alkylation. In contrast to Friedel-Crafts acylations, which have widespread utility, the usefulness of Friedel-Crafts alkylations is limited by a tendancy toward over-alkylation of the aromatic participant, low aromatic regioselectivity and the tendency of carbocation intermediates to rearrange. One of the most important uses of Friedel-Crafts alkylation is ring closure, which is less affected by these limitations than intermolecular reactions are. There is an “intramolecular advantage” associated with generating the carbocation on the same molecule as the aromatic ring. The half-life of the carbocation is decreased by the high local concentration of the reacting partner, which minimizes rearrangement to a more stable secondary carbocation.
Despite the intramolecular advantage, cyclization of MCPA is sluggish because the substitution must occur at a position on the aromatic ring ortho to an electron withdrawing group. An amido group bonded to an aromatic ring through its nitrogen atom, like the amide group in MCPA, is ordinarily a weak activator of the ring toward electrophilic aromatic substitution. However, in the cyclization of MCPA, the amide carbonyl coordinates with AlCl3. Coordination with AlCl3 converts the amide group into an electron withdrawing substituent and deactivates the aromatic ring toward electrophilic substitution. The deactivating effect of a Lewis acid on aromatic ketones has been described in Bull. Soc. Chim. Fr. 1984, 11, 285. In the '637 patent, high temperatures and the highest concentration attainable, i.e. a melt, were used to drive the cyclization onto the deactivated ring of MCPA.
The '637 patent also discloses in Example 11E a process for preparing the Friedel-Crafts starting material MCPA by adding 3-chloropropionyl chloride dropwise to a solution of p-anisidine in dry acetone.
Several investigators working in Japan have described modifications to the Friedel-Crafts reaction conditions of the '637 patent.
According to Chemical Abstracts Doc. No. 127:34142, Japanese Patent No. 9-124605 describes an improved process in which the MCPA and AlCl3 are diluted with a liquid paraffin in mixture with either DMSO or an amide. Suitable amides in the JP '605 process include N,N-dimethylformamide (“DMF”) and N,N-dimethylacetamide (“DMA”). A 76.9% yield of 6-HQ is reported after 20 h at 105° C. in a mixture of paraffin and DMA. Suitable paraffins are C7-C14 hydocarbons. For a large scale process, the lower molecular weight hydrocarbons are preferable for economic reasons. As an example, one liter of the C7 hydrocarbon n-heptane costs less than a tenth as much of the C12 hydrocarbon dodecane.
In our hands and conducting the reaction in n-heptane and DMF at 100° C., the yield was lower than claimed in the JP '605 patent. The reaction also took longer but the purity of the product obtained after recrystallization from methanol and toluene was indeed improved over the '637 process. As is apparent from the Chemical Abstract, this process suffers from a slow reaction rate.
According to Chemical Abstracts Doc. No. 133:585428, Japanese Patent No. 2000-229944, describes the AlCl3 catalyzed cyclization of MCPA in high boiling hydrocarbons like decahydronaphthalene and tetralin, and high boiling ethers like benzyl ethyl ether, isoamyl ether, diphenyl ether, diglyme and triglyme. Reaction of MCPA and AlCl3 for 8 h in decahydronaphthalene at 150° C. gave 6-HQ in 90% yield. The JP '944 patent also discloses a preparation of MCPA from p-anisidine and 3-chloropropionyl chloride in DMF, DMA, DMSO and diphenyl.
According to Chemical Abstracts Doc. No. 131:257448, Japanese Patent No. 11-269148 describes the Friedel-Crafts intramolecular alkylation of MCPA in a mixture of a halobenzene and an amide or amine. The reaction may be performed at between 110° C. and 200° C. with 0.1 to 10 equivalents of amine. It is reported that 6-HQ was obtained in 78% yield after fifteen hours at 130° C. in a mixture o-dichlorobenzene and trioctylamine.
Aromatic solvents like benzene and tetralin are usually a poor solvent choice for conducting Friedel-Crafts alkylation reactions because the solvent, which is usually present in large excess, is susceptible to electrophilic attack. Halobenzenes like o-dichlorobenzene are somewhat deactivated towards electrophilic attack and, as mentioned, there is an intramolecular advantage favoring the Friedel-Crafts alkylation of MCPA. However, since the aromatic ring of MCPA is also deactivated, it was found that reaction with solvent was competitive with cyclization. The side products of reactions with solvent were detected as a complex pattern of peaks in the HPLC chromatogram of the product mixture that was absent from the chromatograms of the product mixtures from the melt and paraffin processes.
It would be desirable to have a process for making 6-HQ in a high level of purity, by a reaction that proceeds at a fast rate and with an improvement in the yield.